Characterization of the microemulsion
The electrical conductivity (κ) of the microemulsion is sensitive to the change of its microstructure.29,30 Therefore, it is usually used to distinguish the subregions in the microemulsion. The variations of electrical conductivity with respect to the mass fraction of oil phase (WOA, octane phase containing 50 mmol L-1 acetic acid) at different mass ratios ofn-propanol to water (RP/W) are presented in Figure S1 and some typical results are shown in Figure 2A. It can be found that the κ value increases initially and then decreases with the increasing of WOA value. For the purpose of clarity, the curve ofκ with the increase of WOA at fixed RP/W of 9.02 is taken as an example alone (Figure 2B). Obviously, there are two breaks at the tested WOA values of 10.4 and 20.8 wt.%, which divide the curve into three stages. At the first stage, the initial increase of κ before the WOA value reaching to 10.4 wt.% is attributed to the successive increase of the conductivity of oil/water (O/W) microemulsion droplets. This behavior can be further assigned to the continuous adding of octane into aqueous phase since the droplets can absorb the ions from water and the charges are generated by friction in the aqueous phase. Thus, the larger conductivity value with the increased WOA indicates the formation of O/W microemulsion. Then, the electrical conductivity begins to decrease with further increase of WOA, showing the generation of bicontinuous (B.C) microemulsion, where the O/W microemulsion droplets are broken by collision, and the oil phase also begins to aggregate to form network-like structures, which can be observed distinctly by the following Cryo-SEM image (Figure 5B). In this stage, an inverse relationship between electrical conductivity and octane concentration is observed. It is because the conductivity of this system depends on the conductive channels generated by the continuous water phase, where these channels became much narrower with the decline of water content. When the WOA value exceeds 20.8 wt.%, the κ value decreases linearly with the increase of WOA, which corresponds to the formation of water/oil (W/O) microemulsion. In this W/O microemulsion region, the continuous phase is octane and the κ value only relies on the movability of water micro-droplets, where the gradual disappearance of water droplets results in a linear decrease of κ value in this system with increasing WOA (more than 20.8wt.%). Thus, the three subregions marked O/W, B.C and W/O of the single-phase region shown in Figure 1 are distinguished clearly through electrical conductivity measurements.